Embodiments of the disclosure relate generally to the field of measuring angular, bending, and linear displacement, by sensors composed of fiber optics that modulate the intensity of light, and more particularly to fiber optic position sensors with a diameter and range of less than a millimeter.
Detection of linear displacement with nanometer scale resolution can be accomplished by a variety of standard position sensors, including piezoelectric, resistive, inductive, capacitive, and optical types, and recent designs have reached sizes, as small as a few millimeters as disclosed in A. J. Fleming, “A review of nanometer resolution position sensors: Operation and performance,” Sensors and Actuators A 190, 106 (2013); U.S. Pat. No. 8,570,529 B2, “Device for position detection”; U.S. Pat. No. 8,773,666 B2, “Device and method for acquiring position with a confocal Fabry-Perot interferometer”; U.S. Pat. No. 7,038,443 B2, “Linear Variable Differential Transformers For High Precision Position Measurements”. However, there are currently no inexpensive, simple, and robust position sensors, with sizes well below a millimeter to use in small instrument designs.
Interferometry based optical displacement sensors modulate the phase of light and achieve high resolution and small sensor size, but are complex and expensive to produce, U.S. Pat. No. 8,773,666; U.S. Pat. No. 6,069,686, “Self-Calibrating Optical Fiber Pressure, Strain And Temperature Sensors”; U.S. Pat. No. 5,477,323, “Fiber Optic Strain Sensor And Read-Out System”. Intensity based optical position sensors modulate the amplitude of light, and a variety of simple and inexpensive designs have been developed. For example, the divergence of light from the end facet of a single-mode fiber can modulate the optical intensity to sense position, U.S. Pat. No. 4,670,649, “Optical Transducer And Measuring Device”; U.S. Pat. No. 5,473,156, “Optical Displacement Sensor Using Dual Reference Reflectors”. Similarly, the assignee of the present application has developed an external sensor that modulates the optical intensity with angular displacement, for scanning probe microscopy; U.S. Pat. No. 9,366,695, “Scanning Probe Microscope Head Design”. Optical intensity can be internally modulated by displacement, using the emission of light from the core of a fiber with surface treatment, however, the size, cost, and resolution of such designs have not been practically demonstrated, U.S. Pat. No. 5,633,494, “Fiber Optic Bending And Position Sensor With Selected Curved Light Emission Surfaces”. The simplest and best way to measure displacements internally in a displaced fiber is by mode cross coupling to change the optical intensity, U.S. Pat. No. 4,342,907, “Optical Sensing Apparatus And Method”.